1. Field of the Invention
The present invention relates to a discharge lamp arc tube and a method of producing the arc tube. More particularly, the present invention relates to a discharge lamp arc tube and a method for producing the arc tube in which molybdenum foil, used in pinch seal portions for providing airtighteness to the glass bulb of the arc tube, has a roughened surface created by etching the foil using oxidation and reduction treatments.
2. Description of the Related Art
FIG. 8 shows a related art discharge lamp. The discharge lamp has a structure in which front and rear end portions of an arc tube 5 are integrated with an electrically insulating base 1 while supported by a lead support 2 and a metal grip member S. The lead support 2 serves also as a current conduction path protruded frontward from the electrically insulating base 1, and the metal grip member S is fixed to the front of the electrically insulating base 1.
The arc tube 5 further has a structure in which a closed glass bulb 5a provided with a pair of opposite electrode rods 6 and 6 and filled with a light emitting substance or the like is formed between a pair of front and rear pinch seal portions 5b and 5b. A sheet of molybdenum foil 7 for connecting the electrode rod 6 protruded into the closed glass bulb 5a and a lead wire 8 led out from the pinch seal portion 5b is sealed in the inside of the pinch seal portion 5b, so that the pinch seal portion 5b is kept airtight.
The electrode rod 6 is most preferably made of tungsten because of that material's excellent durability. However, since the linear expansion coefficient of tungsten is largely different from that of glass, tungsten is unfamiliar with glass and therefore, inferior in airtightness. Accordingly, when the sheet of molybdenum foil 7 having a linear expansion coefficient near to that of glass and relatively familiar with glass is connected to the tungsten electrode rod 6 and sealed at the pinch seal portion 5b, the pinch seal portion 5b can be kept airtight.
Further, ultraviolet-shielding shroud glass G is integrally welded to the arc tube 5. A region from the pinch seal portion 5b to the closed glass bulb 5a is covered with the shroud glass G so that an ultraviolet-ray component having a wavelength region harmful to the human body in light emitted from the arc tube 5 is cut-off. At the same time, the region from the pinch seal portion 5b to the closed glass bulb 5a is surrounded by a closed space formed by the shroud glass G so that the closed glass bulb 5a is kept at a high temperature.
In the related art arc tube, although it can be said that the sheet of molybdenum foil 7 sealed at the pinch seal portion 5b is familiar with glass, it cannot be said that the linear expansion coefficient of the molybdenum foil 7 is quite the same as that of glass. Also the difference between the temperature at the time of switching on the lamp and the temperature at the time of switching off the lamp is large and, therefore, thermal stress is generated in the interface between molybdenum foil 7 and glass with the change of the temperature. Moreover, vibration of an engine or vibration generated with the running of a car is transmitted to the arc tube. Therefore, there becomes a problem that a gap can be formed between the molybdenum foil 7 and the glass material in use for a long term. That is, foil rising occurs which leads to leakage of a sealing substance contained in the closed glass bulb.
Therefore, the present inventor has conceived that such foil rising may be prevented when the adhesion (mechanical bonding strength) between molybdenum foil and glass in each pinch seal portion is enhanced and, accordingly, a surface of the sheet of molybdenum foil is provided as a roughened surface having a micro-asperity shape. It has been then confirmed that foil rising can be suppressed effectively when a sheet of molybdenum foil is subjected to an oxidation treatment and then subjected to a reduction treatment so that a roughened surface having a micro-asperity shape is formed on a surface of the sheet of molybdenum foil and the sheet of molybdenum foil having such a roughened surface is sealed at a pinch seal portion.